This invention relates generally to ignition systems in internal combustion engines and, more particularly, relates to an apparatus and method for utilizing ionization measurement for air/fuel ratio control to reduce engine emissions and increase engine efficiencies.
It is necessary to control the air/fuel ratio introduced into the cylinders of internal combustion engines for many reasons including emissions control, engine efficiency, catalytic converter efficiency, catalytic converter longevity and engine power. Numerous methods and apparatuses exist in the prior art to control the air/fuel ratio especially in light of governmental pressures to reduce certain emissions. Overall control of internal combustion engines is currently premised on the reading of various engine operating parameters such as engine speed, intake manifold pressure, coolant temperature, throttle position, and exhaust oxygen concentration. These parameters are used in conjunction with specific, predetermined base maps calibrated by a baseline engine to select the ignition timing, fuel injector duration, and exhaust gas recirculation ("EGR") of the engine so that the engine achieves maximum efficiency and minimum emissions as determined by the baseline engine.
Present engine control systems, and more specifically, air/fuel ratio control systems, do not adequately control internal combustion engines so that maximum efficiency and reduced emissions are achieved. For example, U.S. Pat. No. 4,543,934 provides a fuel-air mixture dilution control system by monitoring cycle-to-cycle fluctuations of the angular position of peak combustion pressure of each engine cylinder. This control system determines an air/fuel ratio at which engine stability changes between stable and unstable conditions. A controller attempts to continuously operate the engine at the engine stability point, leaning the fuel-air mixture until the engine becomes unstable, and enriching the fuel-air mixture until the engine becomes stable again. This stability point is often beyond the point of maximum efficiency and is also often beyond the point of minimum emissions. Other control systems, such as the system disclosed in U.S. Pat. No. 4,736,724, control the air/fuel ratio by measuring the burn duration of each engine cylinder. The duration is compared to an adaptive engine map that determines the lean limit for the engine at a specific speed and load. The engine is then controlled to operate at the most dilute point possible for a desired engine stability, but this point is often beyond the point of maximum efficiency, and is often beyond the point of minimum emissions. U.S. Pat. No. 4,621,603 discloses three different methods of controlling the level of fuel-air mixture dilution using pressure ratio management. The first system controls the amount of diluent at a specified value as a function of engine speed and load. The second system controls the amount of diluent to adjust the burn rate or combustion time. The third system controls the amount of diluent using cycle-to-cycle variability as both a method to balance fuel delivery to each combustion chamber, and as a method of stability control. Pressure ratio management allows for a simplified algorithm, but again does not supply the engine controller with enough information for complete engine control because taking pressure readings only at specific points allows the controller only to estimate engine stability, and therefore, this system suffers the same limitations of the previously mentioned systems. Alternatively, the system of U.S. Pat. No. 4,621,603 could be used at a specific air/fuel ratio that is calculated according to base maps, but even with an adaptive algorithm, the pressure ratio does not give enough information to allow the system to provide both maximum efficiency and minimum emissions. The system in U.S. Pat. No. 4,621,603, for example, would have extreme difficulty calculating the engine mean effective pressure if spark timing varies by large amounts. Such a calculation is necessary for an engine to achieve maximum efficiency at highly dilute mixtures and minimum emissions.
An important consideration in air/fuel ratio control methodology is catalytic converter performance. In order to optimize catalytic converter performance, a stoichiometric air/fuel ratio (about 14.7 to 1 for gasoline) is desirable. This is because with rich air/fuel ratios (i.e., less than 14.7 to 1) the fuel does not completely combust and the resulting emissions tend to clog the catalytic converter. A lean mixture (i.e., greater than 14.7 to 1), on the other side of stoichiometric, results in excess oxygen ("O.sub.2 ") in the emissions which in turn causes the operating temperature of the catalytic converter to rise and reduces or prevents the conversion of nitrogen-oxygen compounds ("NO.sub.x "). Exposure to elevated temperatures sharply reduces the operating life of the catalytic converter. In sum, catalytic converters are at their most efficient when a stoichiometric air/fuel ratio is used in the engine cylinders.
Most air/fuel ratio control methods use oxygen sensors in the exhaust system of the engine to measure the presence of oxygen which is indicative of whether the engine is running at stoichiometric mixtures. The O.sub.2 sensor measures the O.sub.2 in the exhaust of the engine in either the exhaust manifold or the exhaust pipe. One drawback to using an O.sub.2 sensor in the exhaust manifold or pipe is that the sensor reads a global air/fuel ratio for all engine cylinders. If one cylinder runs lean because, for example, a fuel injector is clogged, an air/fuel ratio controller that is based upon the O.sub.2 sensor will cause the other cylinders to run more richly thereby maintaining the desired global air/fuel ratio. Such a system achieves an average stoichiometric air/fuel ratio for all the cylinders, even though individual cylinders may be running at undesirably rich or lean mixtures.
There have been a number of attempts using O.sub.2 sensors to replace the above-described global emissions control with control of the air/fuel ratios in individual cylinders. The most common method of individually controlling the air/fuel ratio is to utilize fast acting O.sub.2 sensors to discern the exhaust O.sub.2 from each of the cylinders individually. The primary drawback with this implementation is that the O.sub.2 sensors are down-stream from the cylinders. The physical separation between the cylinder where combustion takes place and the sensor which measures the combustion characteristics introduces time delays, error and control difficulties. It is exceedingly difficult to calibrate this type of air/fuel ratio control system to account for the time delay and error at all engine speeds. Additionally, in some current production engines, four or more O.sub.2 sensors are required for this type of control thereby increasing the cost of implementation.
A relatively recent development allows certain in-cylinder combustion characteristics to be monitored. This monitoring technology revolves around electrically analyzing the gases in the cylinder before, during and after combustion. These gases present in the cylinder include free ions which result from the combustion reaction.
The free ions present in the combustion gases are electrically conductive, and therefore measurable by applying a voltage across either an ionization probe or across the tip of a spark plug. The applied voltage induces a current in the ionized gases which can be measured to provide an ionization signal for analysis. For an example of ionization detection using the tip of a spark plug, see "Ignition System With Ionization Detection", U.S. Pat. No. 5,777,216, issued Jul. 7, 1998 which is commonly owned with the present invention and incorporated herein by reference.
There have been some attempts in the prior art to correlate an ionization signal to air/fuel ratios. The prior art strongly suggests, however, that feedback control of the air/fuel ratio in internal combustion engines based upon ionization signal data is impossible. See N. Callings et al., "Ignition Sensors for Feedback Control of Gasoline Engines", SAE Technical Paper Series No. 884711, 1988, pp. 43-47; R.L. Anderson, "In-Cylinder Measurement of Combustion Characteristics Using Ionization Sensors", SAE Technical Paper Series No. 860485, 1986, pp. 113-124.
In view of the foregoing, an object of the present invention to provide an improved control system and method for regulating the air/fuel ratio introduced into the cylinder of an internal combustion engine.
Another object of the present invention is to provide an improved control system and method of controlling the air/fuel ratio in an internal combustion engine based at least in part upon ionization detection.
Yet another object of the present invention is to provide a control system and method for controlling the air/fuel ratio in an internal combustion engine based upon an ionization signal derived from an ionization detection apparatus.
Still another object of the present invention is to provide a method for controlling the air/fuel ratio in an internal combustion engine that is inexpensive and efficient.